We have, therefore, three alternative methods of evaluating forces between two charges. Equation (3.9) gives only the magnitude of the force. Equation (3.11) gives both magnitude and direction using unit vectors, while Eq. (3.13) or (3.14) gives the force in terms of position vectors. In most cases Eq. (3.13) or (3.14) is preferable since it gives both direction and magnitude and does not require explicit calculation of the unit vector. Note that these relations also give the correct forces in the case in Figure 3.2c, where a negative sign is obtained from the product Q1Q2. This negative sign indicates that the forces are in the directions opposite to those in Eq. (3.13) or (3.14).From Eqs. (3.11), (3.13), and (3.14),wealso notethat F12 =-F21.